1. Field of the Invention
This invention generally relates to structures, systems, and methods for monitoring incident beam position in a wafer inspection system.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various times during a semiconductor manufacturing process to detect defects on wafers. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Determining the positions of defects on wafers is also important to facilitate processes such as defect review, during which locations on the wafer at which defects are located are re-visited to generate additional information about the defects. Therefore, if the determined defect locations are inaccurate, the defects must be searched for during review, which will reduce the throughput of the review process. In addition, inaccurate defect locations may reduce the accuracy and usefulness of review if other defects happen to be located at the inaccurate locations thereby confusing the results of inspection and review. Obviously, as the size of defects decreases, the acceptable error in the defect location also decreases. For example, the difficulty of finding defects based on inaccurate defect locations increases as the defect size decreases. Inaccurate defect locations will obviously affect any process that is performed based on defect location information such as defect repair or removal, defect analysis, etc.
One method for increasing the accuracy of defect detection and defect position determination is to accurately calibrate the inspection system prior to inspection of a wafer. For instance, during calibration of an inspection system, the offset of the light beam from the center of the chuck on which a wafer will be located during inspection may be measured. The measured offset can then be used to correct positional information acquired during the inspection. Therefore, some calibration processes do account for drift in the light beam with respect to the chuck. However, since the calibration processes are typically not performed frequently (e.g., since frequent calibration will reduce throughput), any drift in the position of the light beam between calibrations is not measured. Instead, the incident light beam position with respect to the chuck is assumed to be relatively stable between calibrations. Therefore, any drift in the position of the light beam between calibrations will produce error in the reported coordinates of defect locations on the wafer.
Accordingly, it would be advantageous to develop methods, systems, and structures for monitoring incident beam position in a wafer inspection system that do not have one or more of the disadvantages described above.